In commercial container filling or packaging operations, the containers typically are moved by a conveying system at very high rates of speed. Lubrication may be provided by diluting a concentrated lubricant composition with water to form an aqueous dilute lubricant solution (i.e., dilution ratios of 100:1 to 1000:1), and dispensing copious amounts of aqueous dilute lubricant solution to the conveyor or containers using spraying or pumping equipment or by using a undiluted or “dry lubricant.” These lubricant compositions permit high-speed operation of the conveyor and limit marring of the containers or labels.
Conveyor lubricants are constantly evolving in an effort to meet increasing demands from filling and packaging plants. Specifically, the standards that conveyor lubricants have to meet in terms of (1) PET compatibility, (2) the environment surrounding a conveyor line, (3) cost of the lubricant composition and dispensing the lubricant composition, and the cost of using high amounts of water, and (4) the lubricant dispensing system complexity are becoming more rigorous. Silicone based conveyor lubricants have been seen as meeting some of the increased demands, however, there remains a need for even better silicone based conveyor lubricants that do not adversely affect the environment surrounding the conveyor line, that are cost effective from a composition and dispensing point of view, that are compatible with PET materials, and that are not difficult to dispense.
The compatibility of lubricant compositions with poly(ethylene terephthalate) (PET) is recognized as being important in the prior art both in aqueous dilute lubricants and dry lubricants. However, few prior art teachings measure PET compatibility in terms of bottle failure. What is important in regards to PET compatibility is that PET beverage bottles filled with carbonated beverages and exposed to conveyor lubricant solutions do not show failure under storage. By failure it is meant that the filled bottle bursts or leaks and the contents exit from the bottle. The important measure of the PET compatibility of a lubricant formula is the relative failure rate of bottles exposed to the lubricant. In most prior art publications, PET compatibility is judged by the visual appearance of bottles that have been contacted with lubricant solutions under conditions under which bottles typically do not fail. These prior art teachings assume a correlation exists between the visual appearance of bottles and failure rates when there is in fact no correlation between the appearance of bottles and bottle failure rates. Examples described in patent application Ser. No. 11/233,596, titled SILICONE LUBRICANT WITH GOOD WETTING ON PET SURFACES, and examples described in patent application Ser. No. 11/233,568 titled SILICONE CONVEYOR LUBRICANT WITH STOICHIOMETRIC AMOUNT OF AN ACID, present meaningful PET compatibility test results. In examples described in these two documents it is evident that there is no correlation between the visual appearance and the failure rate of bottles that have been contacted with lubricant compositions.
In some patents, PET compatibility is addressed in part by preferring that contact be avoided between the lubricant composition and portions of thermoplastic containers that are prone to stress cracking, for example the amorphous center base portion of the container. However, in actual practice, it is difficult to prevent lubricant compositions from contacting amorphous stress crack susceptible portions of the bottle and it is instead preferred that the lubricant have a high degree of PET compatibility as measured by a PET compatibility test that evaluates failure rate.
Silicone lubricants have been used on conveyors because they were believed to be PET compatible under the prior art understanding of PET compatibility as determined using a visual test as opposed to a failure test. Also, silicone lubricants were desirable because they provided adequate lubricity on conveyor surfaces. Silicone lubricants include a silicone material that is typically part of a silicone emulsion. In addition to the actual silicone material, a silicone emulsion also includes an emulsifier that allows the silicone raw material to go into solution when formulating. The emulsifier is often a surfactant, and it has been discovered in the present invention that some surfactants used in the emulsion may promote stress cracking in PET containers.
As previously discussed, conveyor lubricants may be used as both a diluted lubricant composition or an undiluted or “dry” lubricant composition. Diluted lubricants are advantageous because they are an effective way of lubricating conveyor surfaces while using less of the concentrated lubricant composition. On the other hand, dry lubricants are seen as advantageous because diluting lubricants with copious amounts of water is wasteful, environmentally unfriendly, and costly. The presence of wet surfaces and standing water provides a medium for the growth of microorganisms including bacteria, yeast, and mold. Puddles of excess lubricant solution on floors create a hazard for slipping and falling. By requiring dilution of the concentrated lubricant, dilution errors can occur, leading to variations and errors in the concentration of the aqueous dilute lubricant solution. Dilution of concentrated lubricant compositions on a conveyor line requires use of equipment that increases system complexity, requires additional maintenance, and may fail or function incorrectly. Water used for dilution of concentrated lubricant solutions on site can cause environmental stress cracking of poly(ethylene terephthalate) (PET) bottles. In addition to issues of increased cost, environmental impact, hazards associated with wet surfaces, increased system complexity, and risk of environmental stress cracking, the practice of diluting lubricant solutions at the point of use gives an unsightly and unclean appearance.
“Dry lubes” have been described in the past as a solution to the disadvantages of dilute aqueous lubricants. A “dry lube” historically has referred to a lubricant composition with less than 50% water that was applied to a container or a conveyor without dilution. Methods of applying conveyor lubricants without in line dilution are described, for example, in U.S. Pat. Nos. 6,288,012; 6,427,826; 6,485,794; 6,495,494; 6,509,302; 6,576,298; 6,673,753; 6,780,823; 6,806,240; 6,821,568; U.S. Patent Applications 2004/0029741A1 and 2005/0003973A1; and PCT Patent Application 01/07544.
In spite of the advantages of “dry lubes” and many efforts to utilize them, practice of conveyor lubricant methods which utilize lubricants in a neat form without dilution are not widely practiced and are generally not practiced in connection with PET bottles that are prone to stress cracking. For practical application of “dry lube” technology with PET bottles, two features which have not been found together in the prior art must be provided simultaneously: acceptable PET compatibility of the lubricant composition and practical means of dispensing.
Practical dispensing of conveyor lubricants requires careful control and maintenance of optimal coefficient of friction values between package and conveyor surfaces, as expressed as a coefficient of friction, sliding force, slip value, frictional resistance or similar term. Generally, the objective for lubricant composition formulation and dispensing in prior art patents and published records is to produce the lowest possible coefficient of friction between conveyed packages and conveyor surfaces. In practice this does not result in effective conveying. In a practical implementation of a conveyor lubrication program, it is in fact insufficient to produce the lowest possible coefficient of friction between conveyed packages and conveyor surfaces. Over application of lubricant compositions and unacceptably low coefficient of friction between packages and the conveyor surface can result in decreased system efficiency up to and including complete inability to transport packages. In the case of packages with height to width ratios much greater than 1, such as bottles, an unacceptably low coefficient of friction may result in an excessive number of tipped and fallen bottles. It is preferred to maintain a proper value for the coefficient of friction that is not necessarily the minimum possible value. Within the same conveyor line, the optimum coefficient of friction is different at different locations on the track. For example, lower coefficient of friction values between packages and conveyor surfaces may be required in faster moving portions of the conveyor such as where packages are being conveyed at a high speed in single file or in transition areas where packages move from single file lines to columns that are several packages wide. Higher coefficient of friction values between packages and conveyor surfaces may be required near the end of conveyor lines to provide sufficient back pressure and forward motive force where packages are finally urged into trays, boxes, cartons or the like. It is highly desirable that the lubricant dispensing system be able to provide different values for the coefficient of friction at different locations on the same conveyor line without requiring different concentrations of lubricant. The capability to provide different coefficient of friction values at different conveyor locations with the same lubricant composition is especially important in the case that the lubricant is not diluted with water at the point of use. Different coefficient of friction values at different conveyor locations is necessarily provided by varying lubricant dispensing system parameters such as the volume of lubricant composition dispensed per area per time.
Several patents acknowledge a preference to minimize lubricant use amounts for reasons of cost. For example, U.S. Patent Application 2004/0029741 states that “Dispensing equipment developed for dosing the liquid composition of the invention has been designed to apply the liquid directly to the surface of the conveyor belt. Since relatively expensive neat product is applied, this equipment has been developed such that any spillage of liquid material (e.g. by flowing under gravity away from the treated surface or dripping down onto the floor) is avoided so as to minimize wastage of said liquid.” A dispensing device recommended in U.S. Patent Application 2004/0029741 is a so-called “flicker” non contact applicator. U.S. Pat. No. 6,382,524 pertains to a “flicker” applicator for applying lubricants which comprises a cylindrical brush that is rotatably mounted in a frame and transfers lubricant from a pick up roller to a conveyor surface by a “flicking action.” U.S. Pat. No. 6,688,434 also states a preference to minimize lubricant use amounts for reasons of cost and waste. According to U.S. Pat. No. 6,688,434 “if too little lubricant composition is sprayed, it is expected that there will be insufficient lubricity between the conveyor and the items being transported on the conveyor. If too much of the lubricant composition is sprayed, it is expected that there will be some waste and increased cost.” U.S. Pat. No. 6,688,434 describes an elaborate dispensing apparatus in which gaseous pressure is used to evenly distribute lubricant through a system of high pressure lubricant lines, nozzles, nozzle valves, and spray valves and to actuate individual spray valves. Other patents describe other lubricant dispensing approaches. For example, U.S. Pat. No. 6,102,161 describes a dispensing device in which a liquid lubricant composition soaks a felt cloth which rests on a conveyor surface and is transferred to the conveyor by contact. U.S. Pat. No. 6,576,298 describes apparatus to generate finely divided droplets or particulates of lubricants by contacting a lubricant flow with an air flow. Dispensing systems according to U.S. Pat. No. 6,576,298 describe separate subsystems for the distribution of compressed air and lubricant composition throughout the conveyor system.
Although prior art patents describe equipment that is capable of applying conveyor lubricant compositions with reduced dripping and waste, they do so with apparatus that are too complex and elaborate. Furthermore, prior art methods seek only to minimize use amounts of lubricant compositions and do not teach methods that are effective to provide different values for the coefficient of friction at different locations.
It is against this background that the present invention has been made.